Chemical-mechanical polishing is commonly used in semiconductor fabrication for planarization, fabrication of contact or via plugs, or fabrication of in-laid interconnects. In the polishing process, a polishing slurry that has particles may be used. The polishing slurry may mechanically and chemically etch the surface of the substrate or an overlying layer. Although polishing slurries may be formulated in a ready-to-use form, many polishing slurries are formulated as a concentrated form or formulated to keep components separate. A polishing slurry in a ready-to-use form includes a large amount of water. Shipping costs for water is expensive due to its density.
In some polishing slurries, polishing particles may change the pH of the slurry. For example, a tungsten polishing slurry may include an aqueous solution of alumina particles and an acid. Alumina particles have an iso-electric point that is basic that can make the polishing slurry more neutral when exposed to the acid for a period of time. A tungsten polishing slurry may have a polishing rate after 15 days that is about a quarter of the initial polishing rate when the components are first mixed. A similar effect occurs with a polishing slurry that includes an aqueous solution of oxide particles in a base because oxide particles have an iso-electric point that is acidic.
To overcome those problems, the polishing slurry may be made by batch mixing near the point of use or by mixing components on a platen of a polisher. With batch mixing, the polishing slurry may be mixed in a large tank. These tanks may be about 300-340 liters (80-90 gallons) but may have volumes of several hundred liters. For one specific process that is used to remove oxide from semiconductor substrates, about 200 liters (55 gallons) of polishing slurry may polish around 400 wafers. The batch mixing still has the problem of a non-stable pH that translates into a variable polishing rate.
The polishing rate of a tungsten polishing slurry including an aqueous solution of alumina particles and an acid may be about 3100 angstroms per minute just after mixing, about 2000 angstroms per minute about one day after mixing, about 1100 angstroms per minute about three days after mixing, and levels off at about 900 angstroms per minute starting at about 15 days after mixing. Assuming that a polishing can polish about 10 wafers an hour, 400 wafers would take about a day and a half to polish. If the 400 wafers were polished after mixing, the first wafer would polish at about 3100 angstroms per minute, and the last wafer would polish at about 1600 angstroms per minute. This highly variable polishing rate makes process control difficult. Although the polishing rate stabilizes at a polishing rate of about 900 angstroms per minute, this rate is too low for production.
Batch mixing has other problems. The mixing tank has to be cleaned each time the solution is changed. Also, lifting and transporting the components for the slurry is difficult due to their densities. Further, the fluid within the tank has to be maintained such that it remains mixed and prevents formation of a gel. When the additional components are added, they need to be completely mixed with the polishing fluid that already exists within the tank. Even after mixing, the polishing rate may need to be characterized because the tank now includes a mixture of fresh and old polishing slurries.
A plurality of feed lines may be directed to dispense directly on to a platen within a chemical-mechanical polisher. One problem with this method is that the components may not properly mix, thereby forming localized regions that are more concentrated with one component versus the other.